Question

I’ve been seeing these two parallel phrases since I started C weeks ago, need someone with the knowledge of the C compiler to tell me which leads to better code.

Version1:

char s[]="aString",*sp=&s,c;
while(c=*sp++){
operation(c);
}

Version2:

char s[]="aString",*sp=&s;
for(;*sp;sp++){
 operation(*sp);
}

Alright, I understand that version2 involves some repeating differencing, so is version1 always better than version 2? if not , what are some typical exceptions?

Answer 1

I’d expect those to be exactly the same at any reasonable optimization level – did you try it?

Edit:

I wanted to confirm, so I tried it. Here’s my example program (I fixed your pointer mismatch errors, too):

void operation(char);

void f1(void)
{
    char s[]="aString",*sp=s,c;
    while(c=*sp++) {
        operation(c);
    }
}

void f2(void)
{
    char s[]="aString",*sp=s;
    for(;*sp;sp++) {
         operation(*sp);
    }
}

Compiled with clang at -O3 on my Mac, here’s the object file:

example.o:
(__TEXT,__text) section
_f1:
0000000000000000    pushq   %rbp
0000000000000001    movq    %rsp,%rbp
0000000000000004    pushq   %rbx
0000000000000005    pushq   %rax
0000000000000006    movq    $0x00676e6972745361,%rax
0000000000000010    movq    %rax,0xf0(%rbp)
0000000000000014    movb    $0x61,%al
0000000000000016    leaq    0xf1(%rbp),%rbx
000000000000001a    nopw    _f1(%rax,%rax)
0000000000000020    movsbl  %al,%edi
0000000000000023    callq   _operation
0000000000000028    movb    (%rbx),%al
000000000000002a    incq    %rbx
000000000000002d    testb   %al,%al
000000000000002f    jne 0x00000020
0000000000000031    addq    $0x08,%rsp
0000000000000035    popq    %rbx
0000000000000036    popq    %rbp
0000000000000037    ret
0000000000000038    nopl    _f1(%rax,%rax)
_f2:
0000000000000040    pushq   %rbp
0000000000000041    movq    %rsp,%rbp
0000000000000044    pushq   %rbx
0000000000000045    pushq   %rax
0000000000000046    movq    $0x00676e6972745361,%rax
0000000000000050    movq    %rax,0xf0(%rbp)
0000000000000054    movb    $0x61,%al
0000000000000056    leaq    0xf1(%rbp),%rbx
000000000000005a    nopw    _f1(%rax,%rax)
0000000000000060    movsbl  %al,%edi
0000000000000063    callq   _operation
0000000000000068    movb    (%rbx),%al
000000000000006a    incq    %rbx
000000000000006d    testb   %al,%al
000000000000006f    jne 0x00000060
0000000000000071    addq    $0x08,%rsp
0000000000000075    popq    %rbx
0000000000000076    popq    %rbp
0000000000000077    ret

As you can see, literally identical.